One method of measuring water vapor (total precipitable water, or PW) in the atmosphere
is to use two narrowband near-IR detectors -- one in the water vapor absorption band and the
other nearby, but outside this band. Figure 1 shows the spectral response of two near-IR
LED detectors, along with the solar spectrum in the near IR. (All figures are from:
"An inexpensive and stable LED Sun photometer for measuring the
water vapor column over South Texas from 1990 to 2001" by Forrest M. Mims III, 2002.)
The wavelengths (880 nm and 940
nm refer to the peak emission wavelength. Typically, the peak response wavelength of LED
detectors is lower than their peak emission wavelength. Although the 940-nm LED does not
encompass the entire water vapor absorption band, the ratio of the response of the two
LEDs is still related to PW and can be calibrated against other PW measurement techniques.

A two-LED water vapor instrument is used just like a two-detector sun photometer. The
instrument is pointed at the sun so that sunlight passes through two small apertures and shines
on the detectors. (Alternatively, there can be one aperture, with the sunlight aligned on
one detector at a time, in quick succession.) The two response voltages are recorded along
with the time of the measurements. Figure 2 shows the ratio of photocurrents (converted to
voltages through a transimpedance amplifier) for such an instrument,
as a function of water vapor in two locations -- Mauna Loa Observatory and Galveston, Texas, US.
These two locations encompass a wide range of water vapor values. Based on the assumption that the
values obtained from independent instruments at MLO and Galveston are correct, this graph
constitutes a calibration of the LED device.

Figure 2. Ratio of detector responses as a function of water vapor.

Figure 3 shows a comparison of an LED instrument with water vapor obtained from Solar Light Company's
MicroTops II sun photometer. Although the correlation is already quite good, it could be
improved by a more thorough calibration of the LED instrument.

Because the response of LED detectors is very stable, they are very useful instruments for
building long time series of data. Figure 4 shows a 12-year record of data from Geronimo
Creek Observatory, Seguin, Texas, USA (29.61°N, 97.93°W), for 1990-2001.

Figure 5 shows PW measurements made with an LED instrument at Geronimo Creek Observatory,
compared to data from two other locations in Texas -- Del Rio and Corpus Cristi. The correlation
coefficient should not necessarily be high because these three sites are separated by hundreds
of kilometers. A close examination of these data could be used to investigate why PW values
differ from place to place.

Figure 5. PW at Geronimo Creek Observatory compared to other locations in
Texas.